The long range objectives of this work are to increase the understanding of the biogenesis of messenger RNA (mRNA) in mammalian cells. Our approach has been based on the fact that many, but not all, RNA molecules in the heterogeneous nuclear RNA (hnRNA) and in the cytoplasmic mRNA populations contain common structural features, as is the case for the 3' and 5' termini. Other distinctive structural features in sub-fractions of these RNA populations include: (1) a transcribed oligo A sequence in hnRNA molecues that lack poly A, that is also not found in cytoplasmic mRNA that has also been detected in about 10 percent of the mRNA molecules of HeLa cells. We have recently developed a technique that allows the isolation of the oligo U(plus) mRNA in good yield that depends on the removal of the poly A sequence before hybridization to poly A attached to agarose. The oligo U (plus) mRNA contains both cap 1 and cap 2 species, an observation confirming its cytoplasmic location since cap 2 structures are not found in nuclear RNA. We are examining the translatability, sequence complexity and homology with total mRNA, of the oligo U containing mRNA as well as the location of the oligo U sequence within selected size classes of the oligo U(plus) mRNA. We are investigating RNA processing that includes characterization of transcription, polyadenylation, capping and RNA ligation reactions observed in mouse myeloma nuclei. An RNA ligase-like activity detected in these nuclei that ressembles in its reaction mechanism the RNA ligase present in E. coli after infection with T4 bacteriophage is being characterized.